Process for the conversion of at least the surface layer of an iron article into magnetite and thus prepared articles



United States Patent O PROCESS FOR THE CONVERSION OF AT LEAST THE SURFACE LAYER OF AN IRON ARTICLE INTO MAGNETITE AND THUS PREPARED AR- TICLES Max Otto Vermij, The Hague, Netherlands, assignnr to De Nederiandse Centrale Organisatie voor Toegepast- Natuurwetenschappelijk Onderzoek, The Hague, Netherlands, a corporation of the Netherlands No Drawing. Application June 19, 1951, Serial No. 232,483

Claims priority, application Netherlands June 21, 1950 Claims. (Ci. 1.48-5.35)

This invention relates to a process for converting a piece of ferrous metal into a coherent magnetite article and to the article produced thereby.

In the chemical industry the preparation of apparatus which is sufiiciently corrosion-resistant for many processes is a difficult problem. This problem is particularly acute when making an anode-material for electrolysis, or electrodialysis, especially when chlorine is developed at the anode. The various stainless steels are not sufiiciently corrosion-resistant; precious metals such as platinum are too expensive for commerical purposes, and even platinum gives unsatisfactory results when chlorine is developed during anodic oxidation processes. Also utilization of graphite anodes causes difliculties in many cases, particularly if oxygen is developed together with chlorine, as graphite pulverizes in this case. As a consequence of this pulverization the anodic liquids are contaminated with loosened graphite particles, and great expense is caused by the rapid wearing away of the electrodes.

In such cases magnetite anodes would be resistant, but up to now magnetite could not be prepared in a serviceable form.

In the Dutch patent specification No. 61,770 a process for the preparation of magnetite is described; by this process it is however obtained as a powder. It is stated that magnetite electrodes can be prepared from this powder, but in practice this turns out to be very diflicult, firstly because it is diflicult to press the powder to a coherent body, and secondly because these electrodes have a very great electrical resistance.

It is also often proposed (e. g. in the German Patent specification No. 255,072) to convert iron articles into magnetite. Usually this takes place by heating iron at a temperature of e. g. 9001000 C. in an oxidizing atmosphere. Under these circumstances the iron converts slowly into magnetite. According to this process it is possible to convert a thin layer of the surface of an iron article to magnetite, but such a layer of magnetite is porous and an article made therefrom has little durability. Therefore the practical usefulness of such magnetite electrodes is small.

If a thicker layer of magnetite is formed, other difliculties arise, for example the magnetite has a poor adherence to the iron core, and the formed magnetite is very brittle. Moreover, according to the experience of applicant, blisters are formed if flat iron articles have their surfaces converted into magnetite and the magnetite extends to a depth of more than 0.5 mm. Thus the upper surface becomes uneven, the thickness of the layer is no longer uniform over the whole surface, the structure of the layer is very bad at many places, and the magnetite easily splits off in fiat scales.

According to the German patent specification No. 235,307 an attempt is made to overcome the foregoing ice defects by converting only the inner sides of iron tubes into magnetite. It was thought that the density of the magnetite would be increased by the radial growth and that no blisters would be formed, but experience proved that blisters still were formed, unless tubes of a very small diameter were used. If, however, an attempt is made to convert the outer side of a tube or the inner side of a larger tube into magnetite this cannot be carried out properly according to any of the known methods, provided a layer of magnetite of several mm. is wanted. Under such circumstances, one or more of the mentioned difiiculties, such as poor adherence, formation of blisters and the like always arise during the process of conversion.

Applicant has found now that the unevenness of the layer of magnetite is related to the character of the surface of the starting material and that-at an otherwise usual treatment of iron articlesa very thick layer of magnetite of a uniform structure can be obtained by profiling the surface of said starting material in special Ways. It is even possible to convert these articles completely into magnetite without formation of blisters.

For this purpose it is necessary to break up the flat surface of the iron before the conversion into magnetite. This is preferably accomplished by means of milling grooves into flat articles or by means of cutting such grooves into tubes, with a lathe. These grooves, for example, may be spaced at a distance of several cm. from one another and may have a depth and a width of about 1 mm. if milled into articles of rolled iron. Said grooves may also be made by a chemical treatment (etching), or by means of sandblasting. Also shallow scratches at small distances from each other, provide some of the benefits of the process. Also by boring a great number of small holes, having e. g. a diameter of 2 mm. and a depth of 2 mm., it was possible to obtain a coherent layer of magnetite by the subsequent heating process.

Often it is advantageous to continue the conversion of iron until all the iron is converted into magnetite.

If e. g. a layer of iron, having the thickness of about 5 mm, remains present in the interior of the electrode, the feeding contacts for the electrical current may be mounted thereinto by boring and screw cutting. This has the advantage that the current is substantially conducted through the iron and only has to pass through a thin layer of the magnetite which has a much greater electrical resistance. An electrode with an iron core is also less fragile than an object entirely consisting of magnetite.

It should be mentioned also that the quality of the iron material has an influence upon the quality of the magnetite. Rolled iron is the usual starting material for this conversion; when cast iron is used, every drawback mentioned above, appears to a greater extent than when using rolled iron. It may however be advantageous to start with cast iron articles, as these can be prepared cheaper in special forms than articles from rolled iron. These cast iron articles can be converted into magnetite also, but then there have to be more grooves, or the grooves need to be deeper than in a similar article made from rolled iron.

Example I From among 4 steel cylinders, having a diameter of about 8 cm., obtained from the same tube, one cylinder (A) was provided with grooves both on the inside and on the outside having a depth of about 1 mm., said grooves having spaced distances varying between 4 and 7 cm. in a second cylinder (B) similar grooves were made only in the inner side thereof; in a third cylinder (C) the cuttings were only made in the outer side thereof, and the surface of the fourth cylinder (D) was not changed at all.

Thereafter these 4 cylinders were heated for 10 hours in an electrical furnace in a stream of steam, at a temperature of about 1100 C. After cooling cylinder A turned out to have a very even and coherent layer of magnetite both inside and outside. Only the inner side of cylinder (B) was converted evenly into magnetite and only the outer side of (C); the sides of these cylinders which did not contain recesses showed several large and small blisters, having a depth of 14 mm. The cylinder Showed these blisters at both sides. The thickness of the evenly formed layers of magnetite varied only from 4.5-5 mm.

Example 11 In a steel plate having a thickness of about 8 mm., profiles were milled at one side, having .a depth of about 1 mm. They were made parallel to the longitudinal direction and to the transverse direction, at mutual distances of respectively 4 and .7 cm.

After heating for 10 hours in an electric furnace in a streaming steam atmosphere, at a temperature of i000 C., the side which is not profiled showed blisters which peeled ofi easily; whereas the profiled side of the plate showed an even layer of magnetite, having a good adhesion to the iron core, this layer having a dense, homogeneous and beautifully coherent structure, and a thickness of about 3 mm.

Example III Three similar electrodes were sawn from a cast iron plate having a thickness of 8 mm. In the first electrode (A) grooves were milled into both sides in two directions perpendicular to each other, said grooves having a depth of 1 mm. and .a width of 1 mm., and spaced distances of 3.5 cm. In the second plate (B) grooves were made in the same way and at the same distances, these grooves having the same width as mentioned above, but a depth of 3 mm.; a third plate (C) was not treated before conversion into magnetite.

After a similar heating as described in Example II the plate C was swollen egglike and showed deep blisters in the midst of it; the exterior thickness of the plate where the blisters occurred was '8 cm., and the magnetite was only very slightly coherent. The layer of magnetite of plate A was more coherent, but still small blisters had appeared which peeled off easily. The layer of magnetite of plate B was flawless; the thickness of this layer was however smaller in the same heating conditions than that upon rolled iron; on this plate it amounted to about 2 mm.

Example IV When similar tubes, as described in Example I, are heated in a stream of carbon dioxide, containing not more than about 10% of carbon monoxide, for a time of about 25 hours at a temperature of about 1000" C., the result is about the same as that of heating in a steam atmosphere for about 10 hours at 1100" C.

Example V A fiat plate of rolled iron, having a thickness of about 6 mm. is grooved with 2 sets of grooves on both sides with a milling machine, the grooves having spaced distances of about 5 c m., a depth of 1.5 mm. and a width of 1 mm. This plate was converted completely into a coherent magnetite plate of about 9 mm. thickness, by heating in a streaming steam atmosphere in an electric furnace at a temperature of about 1050 C. for 14 hours.

I claim:

1. A process for producing an article having at least a surface layer of coherent magnetite, which comprises removing metal from a surface of a shaped piece of ferrous metal so as to form recesses into the body of said piece at intervals over said surface, and thereafter heating said surface under oxidizing and high temperature conditions effective to convert the iron into magnetite,

until at least a surface layer of said piece is converted into a continuous coherent layer of magnetite, said converted piece having a thickness of at least 4 mm. and said recesses being formed at a depth and intervals effective to keep the entire magnetite layer firmly integrated with said piece and avoid blistering of the magnetite.

2. A process for producing an article having at least a surface layer of coherent magnetite, which comprises removing metal from a surface of a shaped piece of ferrous metal so as to form recesses at least about 1 mm. deep into the body of said piece at intervals over said surface, and thereafter heating said surface under oxidizing and high temperature conditions effective to convert the iron into magnetite until at least a surface layer of said piece is converted into a continuous coherent layer of magnetite, said converted piece having a thickness of at least 4 mm. and said recesses being formed at intervals effective to keep the entire magnetite layer firmly integrated with said piece and avoid blistering of the magnetite.

3. A process for producing an article having at least a surface layer of coherent magnetite, which comprises removing metal from a surface of a shaped piece of ferrous metal so as to form recesses at least about 1 mm. deep into the body of said metal at intervals of a plurality of centimeters over said surface, said intervals not exceeding about 7 cm., and thereafter heating said surface under oxidizing and high temperature conditions effective to convert the iron into magnetite until at least a surface layer of said piece is converted into a continuous coherent layer of magnetite, said converted piece having a thickness of at least 4 mm.

4. A process for producing an article having at least a surface layer of coherent magnetite, which comprises forming continuous grooves in a surface of a shaped piece of ferrous metal at intervals over said surface, and thereafter heating said surface under oxidizing and high temperature conditions effective to convert the iron int-o magnetite, until at least a surface layer of said piece is converted into a continuous coherent layer of magnetite, said converted piece having a thickness of at least 4- mm. and said grooves being formed at a depth and intervals effective to keep the entire magnetite layer firmly integrated with said piece and avoid blistering of the magnetite.

5. A process for producing an article having at least a surface layer of coherent magnetite, which comprises cutting grooves in a surface of a shaped piece of ferrous metal at intervals over said surface, and thereafter heating said surface under oxidizing and high temperature conditions effective to convert the iron into magnetite, until at least a surface layer of said piece is converted into a continuous coherent layer of magnetite, said converted piece having a thickness of at least 4 mm. and said grooves being formed at a depth and intervals effective to keep the entire magnetite layer firmly integrated with said piece and avoid blistering of the magnetite.

6. A process for producing an article having a coherent surface layer .of magnetite, which comprises removing metal from a surface of a shaped piece of ferrous metal so as to form recesses into the body of said piece at intervals over said surface, thereafter heating said surface under oxidizing and high temperature conditions effective to convert the iron into magnetite until a surface layer of said piece having a thickness of at least 2 mm. is converted into magnetite, and discontinuing the heating while said piece still comprises a core of ferrous metal, said recesses being formed at a depth and intervals effective to keep the entire magnetite layer firmly integrated with said core and avoid blistering of the magnetite.

7. A process for producing a shaped, article having a thickness of at least 4 mm. and composed substantially entirely of coherent magnetite, which comprises removing metal from opposite surfaces of a distended piece of ferrous metal so as to form recesses into the body of said piece at intervals over each of said surfaces, and thereafter heating said surfaces under oxidizing and temperature conditions efiective to convert the iron into magnetite until said piece is converirsd substantially entirely into coherent magnetite, the converted piece having a thickness of at least 4 mm. and said recesses being formed at a depth and intervals efiective to keep the entire converted piece firmly integrated and avoid blistering 0f the magnetite.

8. A process for producing a shaped article having at least a surface layer of coherent magnetite, which comprises cutting grooves at least about 1 mm. deep into a surface of a ferrous metal plate at intervals of a plurality of centimeters over said surface, said intervals not exceeding about 7 cm., and thereafter heating said surface in a mildly oxidizing atmosphere at a temperature of about 1000 to 1100 C. until the iron in at least a surface layer of said plate is converted into a continuous coherent layer of magnetite having a thickness of at least 2 mm., the converted plate having a thickness of at least 4 mm.

9. A shaped rigid article having at least a surface layer or" coherent magnetite and produced by the process described in claim 1.

10. An anode having a core of ferrous metal and a coherent surface layer of magnetite, produced by the process described in claim 6.

References Cited in the file of this patent UNITED STATES PATENTS 24,604 Pomeroy June 28, 1859 513,679 Hemingway Jan. 30, 1894 568,229 Blackman Sept. 22, 1896 705,935 Lee et a1. July 29, 1902 762,227 Blackman June 7, 1904 854,813 Dunwoody May 28, 1907 1,039,071 Davis, Jr Sept. 17, 1912 1,056,627 Carnahan et a1. Mar. 18, 1913 1,977,270 Feild Oct. 16, 1934 2,213,759 Erb Sept. 3, 1940 2,269,943 Kiser Jan. 13, 1942 2,305,539 Lowry Dec. 15, 1942 2,333,936 Johnson Nov. 9, 1943 2,547,536 Pollard, Jr. Apr. 3, 1951 

1. A PROCESS FOR PRODUCING AN ARTICLE HAVING AT LEAST A SURFACE LAYER OF COHERENT MAGNETITE, WHICH COMPRISES REMOVING METAL FROM A SURFACE OF A SHAPED PIECE OF FERROUS METAL SO AS TO FORM RECESSES INTO THE BODY OF SAID PIECE AT INTERVALS OVER SAID SURFACE, AND THEREAFTER HEATING SAID SURFACE UNDER OXIDIZING AND HIGH TEMPERATURE CONDITIONS EFFECTIVE TO CONVERT THE IRON INTO MAGNETITE, UNTIL AT LEAST A SURFACE LAYER OF SAID PIECE IS CONVERTED INTO A CONTINUOUS COHERENT LAYER OF MAGNETITE, SAID CONVERTED PIECE HAVING A THICKNESS OF AT LEAST 4 MM. AND SAID RECESSES BEING FORMED AT A DEPTH AND INTERVALS EFFECTIVE TO KEEP THE ENTIRE MAGNETITE LAYER FIRMLY INTEGRATED WITH SAID PIECE AND AVOID BLISTERING OF THE MAGNETITE. 